a. Field of the Invention
The present invention relates generally to drills and similar boring tools, and, more particularly, to a guide adaptor for facilitating removal of rivets using a hand-held power drill.
b. Background Art
In various types of assemblies, it is necessary to remove rivets from aluminum or sheet metal panels for maintenance, inspection, repair and so on. This is particularly true in the aircraft industry, where literally hundreds of rivets must be drilled out in order to periodically remove alloy panels from wing surfaces, fuselage exteriors, and other parts of the aircraft.
As can be seen in FIGS. 2 and 3, a typical rivet of the type which is used to secure light alloy panels includes a domed upper head portion which engages the outer surface of the panel, and a shaft portion which extends through a bore formed in the panel or panels. At its lower end, the shaft of the rivet is flared or "mushroomed" outwardly into engagement with the inner surface of the panel.
In order to remove a rivet, a hole is drilled in the center of the head and partway along the length of the shaft, leaving a "donut" which is sheared off by applying pressure in a lateral direction using a chisel or similar tool. The shaft then drops or is pushed out of the bore so that the remainder of the rivet falls out on the opposite side of the panel.
While simple in theory, this is in fact a fairly painstaking process. Firstly, the diameter of the drill bit must be quite close to that of the shaft of the rivet, only just slightly undersized, or else insufficient material will be bored out for the head of the rivet to be knocked off with a clean blow; if the head does not come off easily and requires extensive chiseling/hammering, this can easily result in damage to the surrounding panel. At the same time, however, the operator must be very careful that the drill bit does not cut into the material of the panel surrounding the bore; while the bore can be drilled out and fitted with an oversized rivet if damaged, this is inconvenient and inefficient, and for structural reasons only a small amount of enlargement is allowed before the panel must be discarded.
Hence, proper removal requires very precise alignment of the drill bit with the shaft of the rivet. Moreover, the rivet must be drilled to a fairly exact depth, since too shallow a bore will remove insufficient material, whereas a bore which extends too far into the shaft of the rivet will leave a thin annular wall or sleeve of material adjacent the surface of the panel, which will tend to collapse and bend when struck with a chisel, rather than breaking off with a clean shear.
The conventional technique for removing rivets in keeping with these requirements has been to "eyeball" the center of the rivet head, and then mark this with a center punch. The operator then presses the bit of a power tool against the head of the rivet, while carefully holding the drill perpendicular to the panel so that the bit is aligned coaxially with the shaft of the rivet. This is an extremely tedious process, and achieving satisfactory results is highly dependent on operator experience and skill; even then, a skilled operator can easily make a mistake, especially at the end of a long and tiring shift. Consequently, since an average commercial aircraft contains hundreds of thousands of rivets, drilling these out for routine panel removal is an extraordinarily labor intensive and time-consuming process, especially since a single slip of the drill or misalignment can damage a part worth many thousands of dollars.
Over the years, a number of prior rivet drill-out tools have been proposed, such as the examples which are shown in the following U.S. patents: U.S. Pat. No. 1,023,002 (Bennett); U.S. Pat. No. 2,296,087 (Burns, Jr.); U.S. Pat. No. 5,228,811 (Potter); and U.S. Pat. No. 5,318,390 (DalBianco). All of these references show devices having a drill guide and a spring-loaded return mechanism, however none has provided a completely satisfactory solution to the problems noted above. For example, the Bennett patent shows a device in which the cutter rests on the rivet head and is actually larger in diameter than the rivet shaft, for trimming off the edges of the head; while this may have been satisfactory for use with the heavy steel/cast iron plates which were prevalent at the time of the Bennett patent, this device would inevitably cause unacceptable surface damage if used with modern light-alloy panels.
The remaining patents (DalBianco, Potter, and Burns, Jr.) show rivet drill-out tools which are intended to avoid damage to the surrounding structure, but again none of these is entirely satisfactory. For example, in each instance the head of the rivet is received in a generally hemispherical recess in the tip of the tool, with the result that it is very difficult for the operator to keep the drill from tipping out of alignment with the rivet shaft. Also, none of the devices includes a convenient and precise means for adjusting the travel of the bit: In Burns, Jr., depth of the bit is limited by a distance between a shoulder and bearing plate, but this distance is not easily adjustable for bit wear or different sizes of rivet heads; DalBianco, in turn, uses a simple collar and set screw very similar to conventional drill bit depth gauges, which are notoriously prone to slippage and difficult to adjust with any degree of precision.
In practice, the various problems which have been experienced with prior rivet drill-out tools have been so severe that in some instances workers have purposely discarded the devices out of frustration, reverting to their traditional techniques. Moreover, these devices have invariably employed some form of specially-constructed tool body, which has added greatly to their cost. Consequently, the industry has until now been unable to find any satisfactory solution to the problem of quickly, efficiently, and economically removing rivets from lightweight alloy panels.
Accordingly, there exists a need for a rivet drill-out tool which will enable an operator to maintain the bit of a power drill in a stable, coaxial relationship with the rivet as the bit penetrates the head and shaft thereof. Furthermore, there exists a need for such a tool in which the depth of penetration is adjustable in a convenient and precise manner. Still further, there exists a need for such a tool which can be provided as an adaptor for an existing form of tool body, so as to avoid the need for a special tool body and thereby reduce cost of manufacturing the tool.